The inward rectifier K⁺ current (IK1) plays an important role in terminal repolarization and stabilization of the resting potential in cardiac cells. Although IK1 was shown to be sensitive to changes in intracellular Ca²⁺ concentration ([Ca²⁺]i), the nature of this Ca²⁺ sensitivity-in spite of its deep influence on action potential morphology-is controversial. Therefore, we aimed to investigate the effects of a nonadrenergic rise in [Ca²⁺]i on the amplitude of IK1 in canine and human ventricular myocardium and its consequences on cardiac repolarization. IK1, defined as the current inhibited by 10 μM Ba²⁺, was significantly increased in isolated canine myocytes following a steady rise in [Ca²⁺]i. Enhanced IK1 was also observed when [Ca²⁺]i was not buffered by ethylene glycol tetraacetic acid, and [Ca²⁺]I transients were generated. This [Ca²⁺]i-dependent augmentation of IK1 was largely attenuated after inhibition of CaMKII by 1 μM KN-93. Elevation of [Ca²⁺]o in multicellular canine and human ventricular preparations resulted in shortening of action potentials and acceleration of terminal repolarization. High [Ca²⁺]o enhanced the action potential lengthening effect of the Ba(2+)-induced IK1 blockade and attenuated the prolongation of action potentials following a 0.3-μM dofetilide-induced IKr blockade. Blockade of IKs by 0.5 μM HMR-1556 had no significant effect on APD90 in either 2 mM or 4 mM [Ca²⁺]o. It is concluded that high [Ca²⁺]i leads to augmentation of the Ba²⁺-sensitive current in dogs and humans, regardless of the mechanism of the increase. This effect seems to be at least partially mediated by a CaMKII-dependent pathway and may provide an effective endogenous defense against cardiac arrhythmias induced by Ca²⁺ overload.